Frozen Stitch Length on Singer 144W103

[friquant]

How to get this stitch length mechanism freed up?

In this first photo of the main shaft, we have the timing cog on the right, the feed shaft cap in the middle, and the stitch length mechanism on the left. The large hole in the middle of the stitch length mechanism is for a set screw which I have removed. The smaller hole (upper right) of the stitch length mechanism is where I have removed the lock screw. so that the adjustment can actually be made.

Then I rotate the handwheel some, and now we see a different set screw (lower left, already loosened) and the big adjustment screw next to it.

The adjustment screw can move about 200 degrees from left to right before it binds. I've soaked the adjustment screw and the hole where the lock screw goes with marvel mystery oil, and even squirted some between the adjustment mechanism and the feed shaft cap. I've tapped on the adjustment screw with a hammer (using a metal screwdriver as a go-between). I've tapped the feed shaft with a hammer. Still nothing budging.

Is there anything else I should try (or shouldn't try) to get this to move?

 

[friquant]

Here is the relevant section of the manual.

I assume that turning the adjustment screw clockwise is meant to bring the ex-center closer to the center of the shaft.